1. Field of the Invention
The invention relates to a method of stress relieving an age hardenable aluminium alloy product by a permanent cold plastic deformation operation after solution heat treatment and quenching. The invention also relates to a stress relieved product of an age hardenable aluminium alloy, and to uses of such a product.
2. Description of Prior Art
Manufacture of age hardenable aluminium alloy products requires solution heat treatment and quenching of the product. Since residual stresses due to the quenching operation do not allow for machining operations without simultaneous distortion of the machined parts, the products are stress relieved. In case of flat products (e.g. rolled plate) this has been accomplished by a stretching operation using a permanent plastic deformation of a few percent of the original dimension. Usually this stretching is done in the length direction which is normally also the rolling direction.
U.S. Pat. No. 4,294,625 for example describes a process in which aluminium alloy is cast, hot worked into plate, solution treated, quenched, pre-aged, cold rolled to reduce thickness by 11.+-.2%. and then stretched for stress relieving prior to ageing. The product is for use in aircraft. WO 95/24514 similarly briefly mentions stretching a quenched thick aluminium alloy product to improve flatness and reduce residual stress. JP-A-54-102214 describes manufacture of aluminium alloy pipe or rod with low residual stress, by hardening followed by stretch levelling by 0.5-1.0% then roll levelling and further stretch levelling by 0.5-1.0%, followed by tempering at 210-250.degree. C. for 1-2 hours to relieve stress further.
It has also been proposed to employ cold compression as a stress-relieving step. GB-A-2025818 discusses manufacture of aluminium alloy rings by hot ring rolling, solution heat treatment, quenching, cold rolling for stress relieving and ageing. The diameter expansion in the cold rolling is 1 to 3%. Similarly JP-A-3-2359 describes cold compression of a complex shaped hollow conical billet of aluminium alloy, after solution heat treatment and prior to ageing. JP-A-4-187747 describes two-axis cold compression carried out on an aluminium alloy block of complex shape having insert parts located in apertures.
In conventional cold stretching, if the cross-section of the product (plate) is large (e.g. very thick or very wide plate) the strength of the stretcher machine may be insufficient to achieve the desired stretching degree. This of course depends not only on the dimensions of the plate but also on the plate alloy or--more precisely--on the flow stress of the plate material in the solutionized and quenched condition.